Automotive vehicle engines are generally equipped with fuel injectors for injecting fuel (e.g. gasoline or diesel fuel) into the individual cylinders or intake manifold of the engine. The engine fuel injectors are coupled to a fuel rail which contains high pressure fuel that is delivered by way of a fuel delivery system. In diesel engines, conventional fuel injectors typically employ a valve needle that is actuated to open and to close in order to control the amount of fluid fuel metered from the fuel rail and injected into the corresponding engine cylinder or intake manifold.
One type of fuel injector that offers precise metering of fuel is the piezoelectric fuel injector. Piezoelectric fuel injectors employ piezoelectric actuators made of a stack of piezoelectric elements arranged mechanically in series for opening and for closing an injection valve needle to meter fuel injected into the engine. Piezoelectric fuel injectors are well known for use in automotive engines.
The metering of fuel with a piezoelectric fuel injector is generally achieved by controlling the electrical voltage potential applied to the piezoelectric actuators to vary the amount of expansion and contraction of the piezoelectric elements. The voltage is applied to the actuator via positive and negative terminals on the piezoelectric stack. The amount of expansion and contraction of the piezoelectric elements varies the travel distance of a valve needle and, thus, the amount of fuel that is passed through the fuel injector. Piezoelectric fuel injectors offer the ability to meter precisely a small amount of fuel.
Typically, the fuel injectors are grouped together in banks of one or more injectors. As described in EP1400676, each bank of injectors has its own drive circuit for controlling the operation of the injectors. The drive circuit includes a power supply, such as a transformer, which steps-up the voltage generated by a power source, i.e. from 12 Volts to a higher voltage, and storage capacitors for storing charge and, thus, energy. The higher voltage is applied across the storage capacitors which are used to power the charging and discharging of the piezoelectric fuel injectors for each injection event. Drive circuits have also been developed, as described in WO 2005/028836A1, which do not require a dedicated power supply, such as a transformer.
The use of these drive circuits enables the voltage applied across the storage capacitors, and thus the piezoelectric fuel injectors, to be controlled dynamically. This is achieved by using two storage capacitors which are alternately connected to an injector bank. One of the storage capacitors is connected to the injector bank during a charge phase when a charge current flows through the injector bank to charge an injector, thereby initiating an injection event in a ‘charge-to-inject’ fuel injector, or terminating an injection event in a ‘discharge-to-inject’ fuel injector. The other storage capacitor is connected to the injector bank during a discharge phase, to discharge the injectors, thereby terminating the injection event in a charge-to-inject fuel injector, or initiating an injection event in a discharge-to-inject fuel injector. The expressions “charging the injectors” and “discharging the injectors” are used for convenience and refer to the processes of charging and discharging, respectively, the piezoelectric actuators of the fuel injectors.
A regeneration switch is used during a regeneration phase at the end of the charge phase, and before a later discharge phase, to replenish the storage capacitors.
Like any circuit, faults may occur in a drive circuit. In safety critical systems, such as diesel engine fuel injection systems, a fault in the drive circuit may lead to a failure of the injection system, which could consequentially result in a catastrophic failure of the engine. Such faults include short circuit faults and open circuit faults in the piezoelectric actuators of the fuel injectors. Three main types of short circuit fault may occur:
i) a short circuit between the terminals of the piezoelectric actuator; otherwise referred to as a ‘stack terminal’ short circuit;
ii) a short circuit from the positive terminal of the piezoelectric actuator to a ground potential; the positive terminal is also referred to as the ‘high’ terminal, and this type of short circuit is generally referred to as a ‘high side to ground’ short circuit; and
iii) a short circuit from the negative terminal of the piezoelectric actuator to a ground potential; the negative terminal is also referred to as the ‘low terminal, and this type of short circuit is generally referred to as a ‘low side to ground’ short circuit.
Diagnostic systems for detecting short circuit, and open circuit faults in the piezoelectric actuators are disclosed in applicant's co-pending patent applications EP 06251881.6, EP 06253619.8, and EP 06256140.2, the contents of each document being incorporated herein by reference.
However, there remains a need for a robust diagnostic system able to detect the various types of short circuit fault described above at engine start-up, that is at key-on, before the injectors are charged and before an injection event takes place.